Blade for a wind turbine rotor

ABSTRACT

Provided is a blade for a rotor of a wind turbine having a substantially horizontal rotor shaft, said rotor comprising a hub, from which the blade extends substantially radially when mounted, said blade having a chord plane extending between a leading edge and a trailing edge of said blade, a root area closest to the hub, an airfoil area furthest away from the hub, a transition area between the root area and the airfoil area, a first airfoil extending substantially along the entire airfoil area, and a second airfoil separately mounted to the blade, said second airfoil arranged at a mutual distance transverse to the chord plane and extending along the root area of the blades.

This is a Continuation Application of U.S. patent application Ser. No.12/083,638, filed Apr. 16, 2008, which was filed under 35 U.S.C. §371 asa national stage of PCT/DK2006/000582, filed Oct. 17, 2006, which claimspriority to Danish application PA 2005 01451, filed on Oct. 17, 2005,the contents of each of which are hereby incorporated by reference intheir entirety.

TECHNICAL FIELD

The present invention relates to a blade for a rotor of a wind turbinehaving a substantially horizontal rotor shaft, said rotor comprising ahub, from which the blade extends substantially radially when mounted,said blade comprising a chord plane extending between the leading edgeand the trailing edge of said blade, and said blade comprising a rootarea closest to the hub, an airfoil area furthest away from the hub anda transition area between the root area and the airfoil area, and saidblade comprising a single airfoil along substantially the entire airfoilarea.

BACKGROUND ART

Ideally, a blade of the airfoil type is shaped like a typical aeroplanewing, where the chord plane width of the blade as well as the firstderivative thereof increase continuously with decreasing distance fromthe hub. This results in the blade, ideally being comparatively wide inthe vicinity of the hub. This again results in problems when having tomount the blade to the hub, and, moreover, this causes great loads whenthe blade is mounted, such as storm loads, due to the large surface areaof the blade.

Therefore, over the years, the construction of blades has developedtowards a shape, where the blade consists of a root area closest to thehub, an airfoil area furthest away from the hub and a transition areabetween the root area and the airfoil area. The airfoil area has anideal or almost ideal blade shape, whereas the root area has asubstantially circular cross-section, which reduces the storm loads andmakes it easier and more safe to mount the blade to the hub. The rootarea diameter is preferably constant along the entire root area. Due tothe circular cross-section, the root area does not contribute to theproduction of the wind turbine and, in fact, lowers the production alittle because of wind resistance. As is suggested by the name, thetransition area has a shape gradually changing from the circular shapeof the root area to the airfoil profile of the airfoil area. Typically,the width of the transition area increases substantially linearly withincreasing distance from the hub.

It is well known from the aeroplane industry that aeroplanes built withtwo wings, so called biplanes, can normally lift more than an aeroplanewith only one wing. This allows for an increase of the total lift of thewings of the aeroplane without increasing the width of the wings.

This principle is also know in connection with blades for wind turbines,i.a. by manufacturing wind turbines with two or more rotors. CA 2 395612 describes a wind turbine with two co-axial rotors where one rotorrotates faster than the second. GB 758 628 describes a wind turbine withtwo co-axial rotors rotating in opposite directions.

WO 98/31934 discloses a blade designed like with a biplane. The blade isconstructed by means of two parallel spars mutually connected by meansof cross braces. Two streamlined elements are fixed on each spar, saidelements comprising a leading edge and a trailing edges, respectively,together providing an airfoil.

U.S. Pat. No. 5,161,952 discloses a biplane construction for windturbine rotors. The wind turbine is designed so that two straight bladesare connected with the hub of the rotor at a distance from each other.The tips of the two blades are interconnected.

DISCLOSURE OF INVENTION

The object of the invention is to provide a new and improved bladeconstruction.

According to the invention, this object is achieved by the bladecomprising at least a first root segment and a second root segment alongsubstantially the entire root area, said segments being arranged with amutual distance transverse to the chord plane, and by at least one ofthe root segments having an airfoil profile. In this way, the rootsegment having the airfoil profile contributes to the production of thewind turbine.

According to a preferred embodiment of the blade according to theinvention, the chord plane of the blade is twisted in the longitudinaldirection of the blade, where the twist may be up to 80 degrees in thelongitudinal direction of the blade. Typically, the twist is between 60and 70 degrees. Usually, the first derivative of the twist increaseswith decreasing distance to the hub, which means, that the twist of thechord plane K1 in the root area 2 preferably is comparatively high.

According to a preferred embodiment of the invention, the chord plane ofat least the one root segment with airfoil profile is substantiallyparallel to the chord plane of the blade itself in the area closest tothe hub. Preferably, the chord plane of the blade is twisted in thelongitudinal direction of the blade in order to compensate for theincrease in local velocity of the blade with increasing distance fromthe hub, which means that the blade “sees” the wind directiondifferently depending on the radius from the hub. This means that inthis embodiment, the course of the chord plane in the longitudinaldirection of the root area is a continuation of the course of the chordplane in the longitudinal direction of the airfoil area and/or thetransition area.

According to a particular embodiment of the invention, both the firstand the second root segment are provided with airfoil profiles. In thisway, both root segments contribute to the production of the wind turbineand may be designed such that the total contribution from the twosegments corresponds to the contribution of the wide portion of theideal blade.

According to a preferred embodiment, the chord plane of the at least oneroot segment with airfoil profile has a substantially constant width,the chord plane of the transition area becoming wider with increasingdistance from the hub, and the chord plane of the airfoil area becomingnarrower with increasing distance from the hub. Thus, the bladeaccording to the invention has a shape corresponding to conventionalblades, so that existing moulds for blades may be modified in acomparatively simple way in order be able to manufacture the new typesof blades.

According to a particular embodiment, the transition area comprises atleast a first transition segment and a second transition segment, saidsegments being arranged with a mutual distance, as seen transverse tothe chord plane, where at least one of the transition segments isprovided with an airfoil profile. Preferably, the chord plane of the atleast one transition segment with airfoil profile is substantiallyparallel to the chord plane of the blade itself at the transitionbetween the transition area and the air-foil area, and preferably, bothtransition segments are provided with airfoil profiles.

Typically, the total length of the root area and the transition area isbetween 5% and 35% of the total length of the blade, and often between5% and 25% of the total length of the blade or even between 5% and 15%of the total length of the blade.

According to a preferred embodiment of the invention, the first and thesecond root segment are joined at the portion of the transition areaclosest to the hub. In this way, the transition area may have a shapecorresponding to a conventional blade, while only the root area has adual profile.

According to another embodiment of the blade, the first and the secondroot segment merge into the first and the second transition segment,where the first and the second transition segment are joined with theportion of the transition area furthest away from the hub. In this way,the transition area also has a dual structure and thus a potentiallygreater lift.

According to a preferred embodiment, the first and the second rootsegment are joined to form a common mounting area at the portion of theroot area closest to the hub. Preferably, this mounting area has asubstantially circular cross-section. In this way, the blade accordingto the invention may have a mounting flange corresponding toconventional blades so that these new blades fit into existing hubs.

According to a preferred embodiment, the blade is made up of a shellbody made of a polymer material reinforced with glass fibres or carbonfibres. Preferably, the blade is designed as a single whole shell body.

Alternatively, the at least one root segment with airfoil profile mayhave a separately mounted segment. This embodiment is advantageous inthat said segments may be used for existing wind turbines without havingto exchange the blades, e.g. by mounting segments to the root part ofalready installed blades. It is also apparent that not all root segmentsare necessarily secured to the hub.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained in detail below with reference to thedrawing, in which

FIG. 1 shows a top view of an ideal blade of the airfoil type,

FIG. 2 shows a perspective view of a conventional blade of the airfoiltype, and

FIG. 3-9 show different embodiments of the blade according to theinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

FIG. 1 shows an embodiment of an ideal blade 20 of the airfoil type. Theblade is provided with a root part 21 adapted to be secured to a hub ofa wind turbine. The ideal blade 20 is designed such that the width ofthe blade 20 decreases with increasing distance L from the hub.Moreover, the first derivative of the width of the blade 20 alsodecreases with increasing distance from the hub 20, which means that,ideally, the blade 20 is very wide at the root area 21. This causesproblems with respect to securing the blade 20 to the hub. Moreover,when mounted, the blade 20 impacts the hub with large storm loadsbecause of the large surface area of the blade 20.

Therefore, over the years, the construction of blades has developedtowards a shape, where the outer part of the blade corresponds to theideal blade 20, whereas the surface area of the root area issubstantially reduced compared to the ideal blade. This embodiment isillustrated with a dashed line in FIG. 1, a perspective view thereofbeing shown in FIG. 2.

As seen from FIG. 2, the conventional blade 1 comprises a root area 2closest to the hub, an airfoil area 4 furthest away from the hub and atransition area 3 between the root area 2 and the airfoil area 4. Theblade 1 comprises a leading edge 5 facing the direction of rotation ofthe blade 1, when the blade is mounted on the hub, and a trailing edge 6facing in the opposite direction to the leading edge 5. The airfoil area4 has an ideal or almost ideal blade shape, whereas the root area 2 hasa substantially circular cross-section, which reduces storm loads andmakes it easier and more safe to mount the blade 1 to the hub.Preferably, the diameter of the root area 2 is constant along the entireroot area 2. The transition area 3 has a shape gradually changing fromthe circular shape of the root area 2 to the airfoil profile of theairfoil area 4. The width of the transition area 3 increasessubstantially linearly with increasing distance L from the hub.

The airfoil area 4 has an airfoil profile with a chord plane K extendingbetween the leading edge 5 and the trailing edge 6 of the blade 1. Thewidth of the chord plane decreases with increasing distance L from thehub. It should be noted that the chord plane does not necessarily runstraight over its entire extent, since the blade may be twisted and/orcurved, thus providing the chord plane with a correspondingly twistedand/or curved course, this being most often the case in order tocompensate for the local velocity of the blade being dependent on theradius from the hub.

Due to the circular cross-section, the root area 2 does not contributeto the production of the wind turbine and, in fact, lowers theproduction a little because of wind resistance. The idea behind theinvention is therefore to divide the root area 2 and optionally also thetransition area 3 into two or more segments, where at least one of thesesegments has a profile providing lift to this part of the blade 1 andthus contributing to the production of the wind turbine.

FIG. 3A shows a first embodiment of a blade according to the invention,where the blade is seen towards the leading edge 5, while FIG. 3B showsa section along the line BB and FIG. 3C shows a section along the lineCC of FIG. 3A, where section CC is situated at the transition betweenthe root area 2 and the transition area 3. According to this embodiment,the root area 2 is divided into a first root segment 7 and a second rootsegment 8, and the transition area 3 is divided into a first transitionsegment 9 and a second transition segment 10. The two transitionsegments 9 and 10 are joined at the transition between the transitionarea 3 and the airfoil area 4. As a result, there is a gap 17 betweenthe segments. Additionally, the segments may be mutually connected bymeans of stiffening means arranged in the space 17 between the segments.These stiffening means may for example be provided as a gridconstruction of e.g. steel and may further be provided with for examplea so-called drag reduction profile, where the cross-section of theprofile has a symmetrical drop shape. In this way, the wind resistanceof the stiffening means is reduced, and noise immissions may be lowered.

FIG. 3B shows the mounting area of the blade. The profiles of the bladesegments in this area are formed such that they lie within acorresponding conventional blade with circular root part (shown with thedashed line 12). The first root segment 7 is provided with an airfoilprofile in the mounting area comprising a chord plane K1, while thesecond root segment 8 is provided with a profile reducing the windresistance of the segment, without necessarily contributing to theproduction of the wind turbine. The first, root segment 7 merges intothe first transition segment 9 also provided with an airfoil profilecomprising a chord plane K3. The second root segment 8 merges into thesecond transition segment 10 and changes gradually from a profile withreduced wind resistance at the mounting area to an actual airfoilprofile comprising a chord plane K4 at the second transition segment 10.The chord plane K of the blade is usually twisted along the longitudinaldirection of the blade to compensate for the local velocity of theblade. Therefore, the course of the chord planes K1, K3, and K4 are acontinuation of the course of the chord plane K of the blade in theairfoil area 4.

The chord plane K of the blade may be twisted as much as 75-80 degreesin the longitudinal direction of the blade, but typically between 60 and70 degrees. Usually, the first derivative of the twist increases withdecreasing distance to the hub, which means that the twist of the chordplane K1 in the root area 2 is preferably comparatively high.

The first root segment 7 and the second root segment 8 are provided witha number of mounting holes 11 in the mounting area. These holes 11 havethe same position as in a conventional blade with circular root part 12.Thus, the new blade according to the invention may be mounted onconventional hubs and thus replace existing blades during a renewal in asimple manner.

FIGS. 4 and 5 show a second and third embodiment of the invention,respectively, and which are modifications of the embodiment shown inFIG. 3. Therefore, only the differences are discussed here. In thesecond embodiment shown in FIG. 4 the second root segment 8 ¹ is alsoprovided with an airfoil profile comprising a chord plane K2 at themounting area. Furthermore, the profile of the root segment 8 ¹ extendsbeyond the conventional circular profile of the root part 12, as isapparent in FIG. 4B. The figure also shows that the chord planes of thetwo segments may be mutually angled.

According to the third embodiment shown in FIG. 5, the first and thesecond root segments 7 ², 8 ² as well as the first and the secondtransition segments 9 ², 10 ² have symmetric profiles. The profiles areshown with the chord planes parallel to the direction of rotation of theblade, however, the profiles may preferably be angled with respect tothe direction of rotation, where the angle is selected based onmaximising the lift.

As seen from FIG. 5B, both the first root segment 7 ² and the secondroot segment 8 ² extend beyond the circular profile of the conventionalroot part 12. Finally, it should be noted that the root segments of theembodiment shown in FIGS. 4 and 5 as well as the embodiment shown inFIG. 3 each merge into a corresponding transition segment (9 ², 9 ³, 10², 10 ³), said segments joining to form a common profile at thetransition between the transition area 3 and the airfoil area 4.

FIG. 6 shows a fourth embodiment of the blade, where the first rootsegment 7 ³ and the second root segment 8 ³ are joined at the mountingarea 22. As seen in FIG. 6B, the mounting area 22 is circular andcontains mounting holes 11, the blades being secured to the hub by boltsthrough said holes. The blade is thus adapted to conventional hubs andcan thus replace existing blades on already installed wind turbinesduring a renewal.

FIG. 7 shows a fifth embodiment of the blade, where the first rootsegment 7 ⁴ and the second root segment 8 ⁴ are joined at the mountingarea 22, but where the two segments are not joint at a distance to thehub, but instead appear as two separate blade parts with differentlengths. It is also apparent from FIG. 7 that the various root segmentsdo not necessarily have the same thickness.

FIG. 8 shows a sixth embodiment of the blade, where a first, a secondand a third root segment 7 ⁵, 8 ⁵, 13 merge into a first, a second and athird transition segment 9 ⁵, 10 ⁵, 14, respectively. The first and thesecond root segments 7 ⁵, 8 ⁵ as well as the first and the secondtransition segments 9 ⁵, 10 ⁵ are here shown with a shape reducing thewind resistance of these segments, while the third root segment 13 andthe third transition segment 14 are provided with an actual airfoilprofile. However, the profiles may all be provided with profilesincreasing the lift of the corresponding areas and thereby also theproduction of the wind turbine. Of course, the three root segments 7 ⁵,8 ⁵, 13 may be joined at the mounting area just like the embodimentshown in FIGS. 6 and 7.

FIG. 9 shows an embodiment corresponding to the embodiment of FIG. 6,but where the blade is additionally provided with a first separatelymounted blade part 15 and a second separately mounted blade part 16. Theseparately mounted blade parts 15, 16 are mounted on the first and thesecond root segment 7 ³, 8 ³, respectively, by means of a number ofretaining means 18. The blade parts 15 and 16 extend along the root area2 and optionally also along the transition area 3 of the blade. Theblade itself does not necessarily have to have two root segments and/ortransition segments, but may have a form where there is no gap 17, noroot segments and/or transition segments, in which case the root areaand the transition area of the blade correspond to a conventional blade.This embodiment is advantageous in that the separately mounted bladeparts 15, 16 may be mounted without having to exchange the blades onalready installed wind turbines. The retaining means may be formed withe.g. a drag reduction profile for lowering wind resistance and noiseimmissions.

The invention has been described with reference to preferredembodiments. Many modifications are conceivable without therebydeviating from the scope of the invention. Modifications and variationsapparent to those skilled in the art are considered to fall within thescope of the present invention. For example, the embodiment shown inFIG. 9 may have only a single separately mounted blade part. Embodimentsare also conceivable where the various segments have a separate bladeangle regulation.

REFERENCE NUMERAL LIST

-   1 blade-   2 root area-   3 transition area-   4 airfoil area-   5 leading edge-   6 trailing edge-   7 first root segment-   8 second root segment-   9 first transition segment-   10 second transition segment-   11 mounting hole-   12 circular root part-   13 third root segment-   14 fourth root segment-   15 first separately mounted blade part-   16 second separately mounted blade part-   17 gap-   18 retaining means-   20 blade-   21 root part-   22 mounting area-   Kx chord plane-   L longitudinal direction

The invention claimed is:
 1. A blade for a rotor of a wind turbinehaving a substantially horizontal rotor shaft, said rotor comprising ahub, from which the blade extends substantially radially when mounted,said blade comprising: a chord plane extending between a leading edgeand a trailing edge of said blade; a root area closest to the hub; anairfoil area furthest away from the hub; a transition area between theroot area and the airfoil area; a first airfoil extending substantiallyalong the entire airfoil area; and a second airfoil separately mountedto the blade, said second airfoil arranged at a mutual distancetransverse to the chord plane and extending along the root area of theblade.
 2. The blade according to claim 1, wherein the second airfoilfurther extends along the transition region of the blade.
 3. The bladeaccording to claim 1, wherein the root area has a substantially circularcross-section and the transition area has a shape gradually changingfrom the substantially circular shape of the root area to a profile ofthe first airfoil.
 4. The blade according to claim 1, wherein the bladecomprises at least a first root segment and a second root segment alongsubstantially the entire root area, the segments are at a mutualdistance transverse to the chord plane, and at least one of the firstroot segment and the second root segment has an airfoil profile.
 5. Theblade according to claim 4, wherein the chord plane of the at least oneroot segment with airfoil profile is substantially parallel to the chordplane of the blade itself in the root area closest to the hub.
 6. Theblade according to claim 4, wherein the first and the second rootsegments have first and second airfoil profiles.
 7. The blade accordingto claim 4, wherein the chord plane of the at least one root segmenthaving an airfoil profile has a substantially constant width, the chordplane of the transition area widens with increasing distance from thehub, and the chord plane of the first airfoil narrows with increasingdistance from the hub.
 8. The blade according to claim 4 wherein thefirst and the second root segments join at a portion of the transitionarea closest to the hub.
 9. The blade according to claim 4 wherein thefirst and the second root segments merge into the first and the secondtransition segments, and the first and the second transition segmentsjoin with a portion of the transition area furthest away from the hub.10. The blade according to claim 4 wherein the first and the second rootsegments merge into the first and the second transition segments, andthe first and the second transition segments join with the portion ofthe transition area furthest away from the hub.
 11. The blade accordingto claim 10, wherein the portion of the root area where the first andsecond root segments join has a substantially circular cross section.12. The blade according to claim 4 wherein the first and the second rootsegments join and form a common mounting area at a portion of the rootarea.
 13. The blade according to claim 1, wherein the chord plane of theblade twists up to 80 degrees in a longitudinal direction of the blade.14. The blade according to claim 1, wherein the transition area furthercomprises a first transition segment and a second transition segment,the segments arranged at a mutual distance as seen transverse to thechord plane, and at least one of the first and second transitionsegments has an airfoil profile.
 15. The blade according to claim 14,wherein the chord plane of the at least one transition segment having anairfoil profile is substantially parallel to the chord plane of theblade at a transition between the blade transition area and the firstairfoil area.
 16. The blade according to claim 15, wherein the first andthe second root segments merge into the first and the second transitionsegments, and the first and the second transition segments join with theportion of the transition area furthest away from the hub.
 17. The bladeaccording to claim 14, wherein the first and the second root segmentsmerge into the first and the second transition segments, and the firstand the second transition segments join with a portion of the transitionarea furthest away from the hub.
 18. A blade for a rotor of a windturbine having a substantially horizontal rotor shaft, said rotorcomprising a hub, from which the blade extends substantially radiallywhen mounted, said blade comprising: a chord plane extending between theleading edge and the trailing edge of said blade; a root area closest tothe hub; an airfoil area furthest away from the hub; a transition areabetween the root area and the airfoil area; an airfoil substantiallyalong the entire airfoil area; and a first root segment and a secondroot segment along substantially the entire root area, said segmentsarranged at a mutual distance transverse to the chord plane, wherein atleast one of the first and second root segments has an airfoil profileand is a separately mounted segment.